The present invention relates to a method of improving adhesive property of polyimide film, more particularly, to a method of improving adhesive property of polyimide film capable of stably generating distinct adhesive property without affecting adhesive agent, and the invention further relates to polyimide film having distinct adhesive property improved by the method of improving adhesive property.
Normally, polyimide film is used for a flexible printed circuit board (FPC) after being laminated with metallic foil (mainly with copper foil) by using an adhesive agent or processed to a laminated board comprising a film layer and a metallic layer via evaporation, plating or a sputtering process. At that time polyimide film is used for a base film, however, poor adhesive property of the conventional polyimide film surface has raised a problem, and using such polyimide film causes faulty products.
One cause of poor adhesive property of polyimide film surface accounts for plane and smooth surface thereon. In detail, the polyimide film surface is smooth and plane even in the case when stabilizer and additive are mixed after producing film by casting solution and performing condensation polymerization of said solution in the process of producing film. When the film surface is smooth and plane, actual adhesive area between the film surface and adhesive agent is relatively contracted, and as a result, sufficient adhesive strength can hardly be generated.
It is scientifically known that stabilizer and additives which were mixed at the time of producing film preferentially bleed from the film surface produced by solution cast method to form "weak boundary layer" (WBL) and migration of low-molecular polymer such as oligomer as composing substance of weak boundary layer could be accelerated. It is also probable that contaminant such as oily matter generated from a film productive plant or contaminant present in the atmosphere adheres to the produced film to result in the formation of stained layer. There is such a case in which because these layers are easy to exfoliate from the film surface, in the case when adhered with adhesive agent or the like, molecular scale of film cannot be adhered firmly, and these layers become a factor to obstruct adhesive property of the film.
It is general to use polyimide film with various kinds of surface treatment given for improving adhesive property of polyimide film surface. Familiar examples of such surface treatments are flame treatment, alkaline treatment, coupling agent treatment for adhering coupling agent solution onto the film surface or sand blasting process by blasting silica sands or the like against the film surface by applying compressed air or centrifugal force.
Though operation functions of these treatments are not clarified, flame treatment is considered to improve adhesiveness of the polyimide film surface by removing evaporated or gassed WBL or stained layer on film surface and purifying film surface. Also alkaline treatment is considered to improve adhesiveness of polyimide film surface by removing WBL or stained layer from film surface with alkaline solution. Coupling agent treatment is considered to improve adhesiveness of polyimide film surface by introducing reactive radical by adhering coupling agent solution onto film surface. Sand blasting treatment is considered to improve adhesiveness of polyimide film surface by increasing adhering area between film surface and adhering agent by forming the uneven surface, and at the same time removing WBL or stained layer from said surface.
However, there has been such a problem that sufficient adhesive intensity to satisfy demand for materializing higher adhesive effect in recent years cannot fully be achieved by these conventional surface treatments.